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Photocatalytic Properties of TiO2 Nanotubes Doped with Ag, Au and Pt or Covered by Ag, Au and Pt Nanodots
Mihail Enachia, Maria Guixb, Tudor Branistea, Vitalie Postolachea, Vladimir Ciobanua, Veaceslav Ursakic, Oliver G. Schmidtb, Ion Tiginyanua,c
"National Center for Material Study and Testing, Technical University of Moldova, Stefan cel Mare, av. 168, MD-2004, Chisinau, Republic of Moldova bInstitute for Integrative Nanoscience, Leibniz Institute for Solid State and Materials Research (IFW), D-01171, Dresden, Germany cInstitute of Electronic Engineering and Nanotechnologies, Academy of Science of Moldova, Academiei, str., 3/3, MD-2028, Chisinau, Republic of Moldova
Titania nanotube arrays have been prepared by anodic oxidation of titanium foils in an electrolyte solution containing a mixture of hydrofluoric acid, ethylene glycol, and phosphoric acid. The initially amorphous nanotubes were found to crystalize in an anatase phase upon thermal treatment at 500°C. Anatase crystalline phase showed a significant improvement in the photocatalytic properties of the prepared samples, which was evaluated by studying their efficiency towards Rhodamine B dye degradation. Additionally, the effect of doping titania nanotubes with noble metals (e.g. Ag, Pt, and Au), or covering their surface with noble metal nanoparticles, was studied regarding their capabilities towards the photocatalytic degradation of Rhodamine B dye. A positive effect when samples were doped with Ag was revealed.
Keywords: doped titania nanotubes, photocatalysis, dye degradation.
УДК 544.03
INTRODUCTION
For many years the process of anodic oxidation of titanium (Ti) in different electrolytes has been studied due to its potential in obtaining nanostruc-tured titanium dioxide (TiO2). This material is of great interest for many applications, such as solar energy conversions in Gratzel-type solar cells [1-3], as photocatalyst [4-7], including reactions of water electrochemical splitting (photoelectrolysis) [8-10], as an active material in gas sensors [11, 12], as well as in several biomedical applications [13, 14]. A key factor in many of those applications is the possibility of producing a nanostructured material with predetermined shapes, like porous structures, tubes, or spheres. These structures present a large active surface in a very small volume, which contributes to the enhancement of their physical and optical properties. Therefore, nanostructured TiO2 is considered one of the most efficient, low-cost, and highly stable photo-catalysts. Nevertheless, the main drawbacks of this material are its wide bandgap and high recombination rate of charge carriers. One way of overcoming these shortcomings is to produce porous materials doped with catalytic metals, which would enhance the photocatalytic properties of TiO2 nanostructures.
The key processes responsible for both the anodic formation of nanoporous titanium dioxide structures and growing matrices of TiO2 nanotubes seem to be the same. They are determined by the metal oxidation and the pore growth controlled by the chemical dissolution of the oxide produced under a given potential difference.
Titania nanotubes are also of especial interest as future platforms to be used as nanoengines, autonomous entities, which perform a controlled motion in the presence of certain fuels or external stimuli [15-20]. While the first potential tubular nano- and micromotors were generally moving in the presence of hydrogen peroxide and showed great potentialities as microtransporters [21-27] and many other sensing and environmental applications [28, 29], their usage has been highly restricted by the toxic nature of the hydrogen peroxide fuel. One of the most interesting alternatives for the mentioned fuel has been the light-induced microengines, whose movement is promoted by a renewable energy source, and the reactions involved do not generally generate waste products. In addition to all advantages of such a motion principle, titania structures are inherently biocompatible, which makes them the perfect candidates for drug delivery in the human body and for other biological applications.
In this regard, the investigation of photocatalytic properties of TiO2 nanotubes and the related mechanisms, including the study of nanotubes doped with catalytic metals, is of special importance not only for their future implementation as microengines but also for their potential implementation in environmental issues related to the photocatalytic decomposition of various pollutants of interest.
In this paper we describe the influence of thermal treatment, doping, and deposition of noble metal nanoparticles on the arrays of titania nanotubes prepared by anodic oxidation, in addition to the photo-
© Enachi Mihail, Guix Maria, Braniste Tudor, Postolache Vitalie, Ciobanu Vladimir, Ursaki Veaceslav, Oliver G. Schmidt, Tiginyanu Ion, Электронная обработка материалов, 2015, 51(1), 3-8.
catalytic activity of such structures with respect to the degradation of Rhodamine B under UV-light.
PREPARATION OF TUBULAR TIO2 NANOTEMPLATES
Ti foils (Sigma-Aldrich) with the thickness of 0.25 mm and purity of 99.7% were used in electrochemical experiments to prepare porous titanium dioxide structures. The samples were degreased in acetone and methanol in an ultrasound bath, followed by washing in deionized water and drying under nitrogen flow before anodic oxidation.
A mixture of hydrofluoric acid (48%), ethylene glycol, and phosphoric acid (H3PO4) in the ratio of 0.95:110:11 ml, respectively, was used for the electrochemical oxidation procedure. The experiments were carried out at room temperature. The Ti foil samples were attached to a Cu support by means of silver paste and mount in a three-electrode electrochemical cell with a Pt-electrode, used as a cathode, while the sample was subjected to electrochemical treatment as an anode, and a Ag/AgCl (1M KCl) electrode was used as a reference electrode. The applied voltage between the working electrode and the reference electrode, as well as the anodic oxidation current, were measured by using a Keithley digital multimeter. A Vegatescan TS5130MM scanning electron microscope (SEM) equipped with an EDX detector was used for sample characterization in terms of morphology and composition.
The oxygen transfer in organic electrolytes is much more difficult compared to that in aqueous electrolytes, which reduces the rate of oxide formation. Therefore, the oxide-reduction process due to the ionic exchange is accelerated in presence of water by the small thickness and the low quality of the formed barrier layer. The incorporation of organic components as electrolytes in the oxide film formation leads to a decrease of the relative permittivity of the layer, contributing to an increase of the breakdown voltage [30]. A high breakdown voltage of the oxide in anhydrous electrolytes ensures a larger range of anodic oxidation voltages, inducing the creation of nanotubular membranes.
The applied voltage was gradually increased at a rate of 1V/s till reaching the value of 120V. Subsequently, that voltage was maintained for different periods of time. The application of a gradually increased voltage ensures the formation of seeds, around which nanotubes are formed afterwards.
Au, Ag, and Pt were initially deposited on Ti foil for the fabrication of doped TiO2 nanomembranes. The deposition was performed by using a Cressing-ton 108 Sputter Coater in a cathodic arc plasma process, achieving the thickness of 500 nm of the catalytic metal films on three different Ti foils. The thicknesses of the deposited metal films can be
precisely controlled by the deposition time. The foils with the deposited metal film were subsequently subjected to thermal treatment under Ar atmosphere at 950°C during 2 hours to promote the diffusion of the catalytic metal into the Ti foil substrate. As a result, TiO2 layers doped with Au, Ag, or Pt were formed on the surface of the substrate. Moreover, arrays of TiO2 nanotubes were obtained on the mentioned foils by means of electrochemical treatment, as described above. Figure 1 presents SEM images of the obtained TiO2 layers.
Fig. 1. SEM images of TiO2 layers produced by anodic oxidation.
However, so as to produce metal nanodots onto the surface of TiO2 nanotube arrays it is necessary to apply different experimental conditions. Fifty nm of Au, Ag, or Pt films were deposited on the surfaces of preliminarily produced TiO2 nanotube arrays to obtain such metal nanostructures. Afterwards the samples were annealed at 500°C to form metal nanodots on the surface of TiO2 nanotubes arrays. The synthesis of metal nanodots is due to the surface tension created during this process, which also promotes the crystalline phase transition from amorphous to anatase structure. It is necessary to mention that annealing at 500°C is enough for the formation of nanodots on the surface of TiO2 nanotubular structure, as illustrated in Fig. 2. One can not subject the produced TiO2 nanotubes to annealing at 950°C, since annealing at temperatures higher than 800°C leads to the formation of a rutile structure with a lower photocatalytic activity. Moreover, upon annealing at 950°C, the tubular structure is deformed.
Fig. 2. SEM images of TiO2 nanotubes covered by catalytic metal nanoparticles.
200 400 600 800 Raman shift, cm-1
Fig. 3. Raman spectra of TiO2 nanotubes as prepared (1) and after being subjected to annealing at 500°C (2).
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Fig. 5. Dependence of absorption upon exposure time for samples with anatase structure doped with Au, Ag, or Pt (a); and for samples covered by metal nanodots (b).
The transition from amorphous to the anatase state upon thermal treatment at 500°C is demonstrated by the Raman scattering analysis, as shown in Fig. 3.
The analysis of Fig. 3 suggests that as prepared TiO2 nanotubes are amorphous, while those annealed at 500°C have an anatase crystalline structure. Anatase is tetragonal, with two TiO2 formula units (six atoms) per primitive cell. The space group
is D4h19 (I4/amd). There are 15 optical modes for this
structure: 1A1g +
+ 3Eg + 2Eu
1A2u + 2Big + 1B2U Three modes are infrared active, namely, the A2u mode and the two EU modes. The B2u mode is silent. The remaining six modes corresponding to symmetries A1g + 2B1g + 3Eg are Raman active. The Raman shift for these phonons is 514 cm-1 for the A1g mode, 399 cm-1 and 514 cm-1 for the B1g modes, and 144 cm-1, 197 cm-1 and 639 cm-1 for the Eg modes
[31]. Therefore, the A1g and one of the 51g modes overlap.
INVESTIGATION OF TUBULAR TIO2 PHOTOCATALYTIC PROPERTIES
The photocatalytic properties of the produced samples have been evaluated by studying the Rhodamine B degradation under UV in the presence of 1 mg of TiO2 nanotubes. The initial concentration of Rhodamine B was 10-6 M, while the illumination source used in such experiments was a LAX-Cute Xenon Light Source 100 W ultraviolet lamp. A fixed volume from each sample has been taken from the solution each hour to perform spectrophotometric studies and to evaluate their absorption at the wavelength of 550 nm. The photodegradation of Rhodamine B by the photocatalytic activity of TiO2 with different crystalline phases is shown in Fig. 4a. A significant photocatalytic activity is observed only for the anatase crystalline phase.
The photodegradation of Rhodamine B, assessed from the decrease of optical absorption, is only appreciable in the case of nanostructured TiO2 sample with the anatase crystalline phase. The absorption values do not practically change upon 5 hours of exposure in the case of the control sample (no nanostructured TiO2 is present in the mixture) or the amorphous nanostructured TiO2 sample. The sample with the anatase structure becomes almost transparent upon 5 hours of exposure to UV-light, while the pink color of the control and of the amorphous samples does not practically change upon irradiation.
The results of investigations concerning the efficiency of Rhodamine B degradation for TiO2 arrays doped with catalytic metals or covered by metal nanoparticles are shown in Fig. 5. The results are compared with those of the control samples (rhombic symbols) as well as with those of the TiO2 undoped samples (square symbols). One can easily realize that there is dye degradation in both doped and nanodot covered samples. However, certain enhancement of the photocatalytic activity regarding Rhodamine B degradation is observed only for the TiO2 nanotube arrays doped with Ag, while doping with Au and Pt, as well as covering of samples with any nanoparticles of Ag, Au, or Pt, leads to a decrease of photocatalytic activity.
It is known that doping or covering of TiO2 samples with noble metals alters their photocatalytic activity. Of special importance are the photoreaction conditions kept during the experiment, as well as the methods and conditions of sample preparation [32]. The effects of doping upon photocatalytic properties can be described in the following terms: (i) an enhancement of the electron-hole separation by acting as electron traps and improving the charge transfer, (ii) an extension of the light absorption into
the visible range and an enhancement of the surface electron excitation due to the excitation of plasmon resonances, (iii) a modification of surface properties of the photocatalyst, (iv) blocking the reaction sites on the TiO2 surface, (v) passivation of the photocata-lyst surface. While the first two effects should improve the photocatalytic properties, the last three can significantly decrease the photocatalytic activity. Therefore, one can conclude that the first two effects prevail in our samples doped with Ag, while the negative effects are predominant in the samples doped with Pt or Au, as well as in samples covered by metal nanoparticles. However, it is not excluded that the optimization of other technological conditions may lead to a positive result for all of the samples.
CONCLUSIONS
The results obtained in the present study demonstrate that thermal treatment of TiO2 nanotube arrays prepared by anodic oxidation of titanium foils at 500°C leads to the transformation of the as-prepared amorphous nanotubular structure to a crystalline anatase phase, which leads to an enhancement of the photocatalytic activity towards the degradation of Rhodamine B dye. Doping of anatase TiO2 nano-tubes with Ag further improves the photocatalytic properties, while doping of samples with Pt or Au, as well as covering with metal nanodots, was found to have a negative impact on the rate of photocata-lytic degradation of Rhodamine B. However, with the corresponding optimization of technological conditions, other positive results related to doping of TiO2 nanostructures by other noble metals could probably be obtained. Additionally, a high photo-catalytic activity shown by the anatase crystalline structures developed in the present work suggests their possible biological application as microengines moved by means of the UV-light, as well as their usage in sensors or as useful tools for environmental purposes.
ACKNOWLEDGEMENTS
Fruitful discussions with Prof. Dr. Vladimir Fomin are gratefully acknowledged. The work has been supported by the bilateral BMBF-ASM project no. 01 DK13010 TiNaTEng.
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Received 06.06.14 Accepted 17.07.14
Реферат
Массивы нанотрубок диоксида титана были получены путем анодного окисления титановой фольги в растворе электролита, содержащего смесь фтористоводородной кислоты, этиленгликоля и фосфорной кислоты. При термообработке при 500°С, первоначально аморфные нанотрубки кристаллизуются в нанотрубки с фазой Анатаз. Нанотрубки с кристалли-
ческой фазой Анатаз показали значительное улучшение фотокаталитических свойств полученных образцов, которые оценивались путем изучения их эффективности с деградаций красителя Родамина Б. Кроме того, эффект легирования нанотрубок диоксида титана благородными металлами (то есть Ag, Р1 и Аи), или покрытия их поверхности благородными металлическими наночастицам, был изучен в отношении их способностей к фотокаталитической деградации Родамина Б. Был выявлен положительный эффект легирования образцов Ag.
Ключевые слова: легированные нанотрубки диоксида титана, фотокатализ, деградация красителей.
